1. Field of the Invention
This invention relates to the field of electrostatography, and more particularly, to improvements in a method and apparatus for controlling toner replenishment.
2. Description of the Prior Art
Toning or development stations for electrostatographic apparatus such as electrophotographic (EP) copiers and printers typically have two-component developer mixtures (carrier and toner). Toner depleted by toning latent images on the photoconductor must be replaced by replenishing with new toner, so that the toner concentration (TC) remains within a usable range in the toning station developer mix.
Closed-loop toner concentration control, for example see U.S. Pat. No. 4,875,078, is typically achieved by means of a TC monitor and control logic to drive a toner replenishment mechanism. TC monitors are of several types, including optical and magnetic. As noted in U.S. Pat. No. 5,649,266, one common practice is to adjust the monitor output, V.sub.MON, to 2.500V when a new load of developer at nominal 10% concentration is installed in the development station. The replenishment algorithm then acts to regulate V.sub.MON to this initial 2.500V value. Maintaining V.sub.MON =2.500V assures that TC=10% (barring monitor drift) regardless of TC monitor sensitivity.
For a given developer at a particular point in its life, as toner charge increases the toner charge to mass ratio (Q/m) decreases, and vice versa. Best system performance is observed when the toner Q/m is within a particular range. An example of a range for one application may be the range of 17-23 .mu.C/g. Problems arise when the ratio of Q/m migrates above or below the particular range suited for that particular application. Examples of problems that occur when the Q/m ratio is low are the tendency towards excessive dusting, hollow character, and post transfer image disruptions. The term "dusting" implies uncontrolled migration of toner to other parts of the apparatus outside of the development station as distinguished from controlled movement of toner to the primary image-forming member to develop the electrostatic image. It is also found that at a given Q/m ratio, higher toner concentrations will give elevated levels of print background.
Typically, problems with Q/m ratios that are low arise through aging of the developer. However, the art has recognized that outright replacement with new developer is an undesirable expensive solution. Accordingly, one known approach for overcoming this problem is described in U.S. Pat. No. 5,678,131 (Alexandrovich et al.). In Alexandrovich et al., a process control procedure is described for regulating toning contrast (D/V). At intervals in an EP process, process control patches are exposed and toned so that an electrometer and a densitometer may read the parameters of voltage and density on the photoconductor. Toning contrast is then computed and compared to the desired value for toning contrast. When the developer is relatively fresh, there is a direct relationship between TC and D/V and the TC setpoint can be adjusted according to the error in D/V. Alexandrovich et al, however, indicates that the rate of adjustment of the TC setpoint is made to be limited so that the change in TC is very gradual. Short term variations in density can be controlled by immediate adjustments in charging and/or exposure while long-term changes are compensated by the gradual adjustment of TC. Further adjustment is made to the rate of adjustment of TC to compensate for the aging of the developer. The problem with this approach is the requirement of multiple readings to infer the toning contrast. The approach can be fairly complex to implement into an electrophotographic apparatus. Each of the measurements will inherently have some error in the desired relationship between TC and the optimum D/V. This approach also requires a toner monitor sensor with a large dynamic range to enable sensing of TC over a large range with adequate linearity.